1. Field of The Invention
This invention generally relates to an image processing and more particularly to an image filtering system suitable for predicting a quantity of codes included in a frame, for example, in case of performing compression of image data representing a color image in blocks, each of which is composed of a predetermined number of picture elements.
2. Description of The Related Art
Generally, when effecting compression of image data representing a color image, the image data are first partitioned into blocks each of which is comprised of a predetermined number of picture elements. Further, data of each block are then transformed by using an orthogonal transform. Subsequently, the transformed data are quantized. Thereafter, either zero run length coding or Huffman coding is performed on the quantized data. Alternatively, both of zero run length coding or Huffman coding are performed on the quantized data.
In case of performing compression of data representing a moving picture, a conventional image filtering system employs the following method. Namely, in case of this conventional method, for example, as illustrated in FIG. 4, one segment is made up of a plurality of frames (four frames in this case). Further, in this conventional method, a frame (hereunder referred to as an intra-frame) on which intra-frame compression is performed is employed as one of the four frames of a segment. Moreover, three frames (hereunder referred to as first to third inter-frames), each of which is composed of difference data representing differences between picture elements of a motion-compensated frame and corresponding picutre elements of the intra-frame, are employed as the other frames of the segment.
In case of performing compression of data of frames in this manner, a quantity of information regarding frequency (i.e., a quantity of codes) included in each frame is predicted by performing a filtering processing of data contained in each frame and computing a sum of absolute values of results of the filtering processing. Then, a quantization step and a band-limited value which correspond to the predicted value of the quantity of codes are established. Subsequently, a band-limitation of the data included in each frame is effected by using the established band-limited value. Thereafter, the band-limited data are orthogonally transformed in blocks, and further the transformed data are quantized by using the established quantization step. Thereby, a quantity of codes included in an entire segment can be controlled in such a manner to be a desired constant value.
However, for instance, in case where an object, which is not present in a previously processed frame, appears in a currently processed frame when performing the filtering processing, or in case where a motion of an object is complex, motion compensation cannot appropriately be performed in the inter-frames. A block, of which motion compensation is not appropriately performed, is not closely correlated with surrounding blocks. Consequently, when reproduction of an image is effected from data obtained by compression of the frame including such a block, there occurs a noise (hereunder sometimes referred to as a block noise) originating from such a block. More particularly, this results from the fact that when computing a sum of absolute values of data obtained as results of the filtering processing of data of such a block, a part of the thus obtained data corresponding to boundary picture elements of the block, which are not used in the orthogonal transform to be performed later on the data of each of the blocks, adjacent to another block to next be read is included in a computed value of the sum. This further increases a prediction error of a quantity of codes obtained as a result of the coding of the data of such a frame.
The present invention is accomplished to resolve such problems of the conventional image filtering system.
Accordingly, an object of the present invention is to provide an image filtering system which can significantly decrease a prediction error of a quantity of codes.